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Severe Storms. Most Violent weather is associated with low pressure systems because air of different properties mixes there Thunderstorms Tornadoes Hurricanes Winter Storms. Thunderstorms. Flash Flooding Hail Lightning Downbursts Tornadoes. Flash Flooding. Lightning. Thunder.

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Severe Storms

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Severe Storms

Most Violent weather is associated with low pressure systems because air of different properties mixes there

  • Thunderstorms

  • Tornadoes

  • Hurricanes

  • Winter Storms


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Thunderstorms

  • Flash Flooding

  • Hail

  • Lightning

  • Downbursts

  • Tornadoes


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Flash Flooding


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Lightning


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Thunder


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Thunder and Lightning

  • Superheated air expands and creates shock wave

  • Can be heard up to 30 miles away

  • Flash-Sound Interval: 5 sec/mi (3 sec/km)

  • Notone second = one mile

  • “Heat Lightning” is ordinary lightning illuminating the clouds


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Lightning Rods

  • Benjamin Franklin, 1752

  • Do not allow lightning strikes to be conducted to the ground

  • Pointed shape allows excess charge to bleed harmlessly into the atmosphere (corona discharge)


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Mammatus


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Squall Line


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The Fujita Scale

Based on Damage and Engineering Studies

  • F040-73 mph29%

  • F174-112 mph40%

  • F2113-157 mph24%

  • F3158-206 mph6 %

  • F4207-260 mph2 %

  • F5261-318 mph<1 %

  • F6?How to identify?


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Conditions for Tornado Formation

  • Energy Source (convection or uplift)

    • Cold Front and Squall Line

    • Supercells and Mesocyclones

  • Vorticity (something to create a spin)

  • Usually but not always spin according to Coriolis Effect

  • Spin is indirectly connected - inherited from larger weather systems


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Mesocyclone


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Mesocyclone, Door County, August 2007


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Door County Tornado, August 1998


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Langlade County Tornado 2007


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Where Tornados Occur

  • U.S. and Canada probably have most severe storms

  • Cool Canadian air meets warm, moist Gulf air

  • Highest reported frequency by area is Britain

  • Other places: India, Australia, China


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Tornado Myths

  • Take shelter in the southwest corner

  • Take shelter under a bridge or overpass

  • Open windows to equalize pressure

  • Buildings explode from pressure drop

  • Tornados avoid rivers, hills, mountains

  • Certain localities are “protected”

  • Tornados avoid cities

  • Should you attempt to evade?


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Fort Worth, Texas, March 28, 2000


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Tornadoes do not avoid cities


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Things Often Mistaken For Tornadoes

  • Heavy Precipitation

  • Downbursts

  • Dust Devils

  • Cold Funnels

  • If There’s No Evidence of Rotation, It’s Not a Tornado


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Virga


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Downburst, May 1994


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Downburst Damage, Ontario


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Dust Devil


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Cold Funnels


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Cold Funnels


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Hurricanes

  • Hurricane: Atlantic and East Pacific

  • Typhoon: West Pacific

  • Cyclone: Indian Ocean

  • Intense Low-Pressure Systems

  • Need 60 m (200 feet) of ocean water at 26.5 C or warmer to form


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World Hurricane Tracks 1995-2003


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Hurricane Forming Regions


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Hurricane-Free Regions

  • No Coriolis effect at equator, hence no hurricanes within 5 degrees of equator

  • No warm sea water in South Atlantic, hence no South Atlantic Hurricanes

  • No warm sea water in Southeast Pacific, hence no Southeast Pacific Hurricanes

  • Apart from Caribbean coast, no hurricanes in South America (maybe?)


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March 2004: Brazil’s First Hurricane?


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Coriolis Effect at Equator


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Coriolis Effect at Equator


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Coriolis Effect at Equator

  • Westbound: Deflected away from Equator

  • Eastbound: Directed along Equator

  • Unlikely for winds but does happen in oceans (Equatorial Countercurrent)

  • Weather systems can’t spin


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Saffir-Simpson Scale

Defined by instruments

  • 74-95 mph1-2m storm surge

  • 96-110 mph2-3 m

  • 111-130 mph3-4 m

  • 131-155 mph4-6 m

  • >155 mph> 6 m


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Naming Hurricanes

  • No naming system until 1953

  • Women’s names 1953-79

  • Regional Name Lists

  • Lists maintained by World Meteorological Organization

  • Names can be retired after especially significant storms


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Naming Hurricanes


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Dangers of Hurricanes

  • Wind Pressure

  • Flying Debris

  • Storm Surge

  • Flash Flooding

  • Tornadoes


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Eye of Hurricanes

  • 100 km or less in diameter

  • 30 minutes or so calm weather

  • Definitely not the end of the storm!

  • Post-eye storm is stronger

  • “Centrifugal” force counteracts inward air flow

  • In strongest storms, air flow can get so congested a second eyewall forms (Andrew)


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Trailing Side is Most Dangerous


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Decay of Hurricanes

  • Need warm water for energy

  • Decay rapidly over land

  • Lose strength over cold water

  • Can still cause destructive flooding long after cyclonic structure is gone

  • Degenerate into low pressure systems


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Cold Water Trail


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Extratropical “Hurricanes”


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Two-Ocean Hurricanes


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Winter Storms

  • Blizzard = Blowing snow and reduced visibility

  • Main Hazards

    • Vehicle accidents

    • Hypothermia

    • Exertion

    • Immobility


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Great Blizzards

  • Schoolhouse Blizzard, Dakotas-Nebraska, Jan. 12, 1888: 235 killed

  • Great Blizzard, East Coast, March 12, 1888: 400 killed, 200 ships sunk

  • Armistice Day Blizzard, upper Midwest, Nov. 11, 1940: 154 killed

  • Storm of the Century, March 12, 1993: Eastern U.S.: 270 died and 48 missing at sea


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